The Argon8? Lots of things...
1) Form factor. This thing's only about the general size of a Roland SH-101, but it's way more potent. Makes it a live MUST.
2) The rev.2 firmware. There's a few interesting surprises in there that really let you dive into the wavetable architecture, and more besides. Still exploring that...
3) Not as gimmicky. One screen for the UI, which is ultimately less distracting. Of course, this means that you lose the ribbon and the poly aftertouch, but I already have a fine old CS-80 for satisfying that itch. Or there's also the CME Xkey25 here, which is part of my multitrack DAW rig. And it's worth noting that the Hydrasynth's poly aftertouch feel was rather different from the CS-80, which I consider the "gold standard" for that, and that was offputting.
4) Modal apparently doesn't have total a-holes working as their regional reps. Also, requests for information from Modal don't result in "thank you for interesting in our suplime product of musical"-type form emails which convey less than zero info, which then necessitate further inquiries that wind up triggering one of the aforementioned a-holes.
5) I actually feel like the Modal does a better job at being closer to a pure wavetable machine than the Hydrasynth, and some of that is clearly due to a smarter UI. It's simple...just like the good ol' PPG Wave 2.3 I had was, and makes a nice complement to the Sledge 2.0, which is also VERY PPGish but...different.
And the clincher...6) It can take a dive to the floor from about 3 1/2 feet up and not only still work, but look as if nothing happened to it at all. Had a bit of an accident while reconfiguring the Jaspers vs Ultimate frankenstand, didn't faze the Argon8 one bit! To say the least, I'm impressed.
The Argon8 does require a good bit of getting used to, true. But so does the Hydrasynth. I'd just prefer to vote with my money as to what I'm willing to put up with out of a manufacturer, and Modal gets that.